The present invention relates in general to methods of measuring high temperatures, and in particular, to a new and useful apparatus and method of measuring high temperatures which utilizes acoustic waves which are transmitted through a medium in a region of high temperature to be measured, and which are received on an opposite side of the region. Since the speed of the acoustic waves through the medium varies with temperature, a measurement of the speed of the acoustic waves can be used to measure the average temperature along the path of the acoustic waves.
The concept of measuring temperature based on the propagation speed of acoustic waves, here referred to as acoustic pyrometry, was first suggested by A. M. Mayer in Phil. Mag., 45, 18 (1873). Instrumentation to accomplish this measurement was not available at that time, however. An article by S. F. Green and A. U. Woodham ("Rapid Furnace Temperature Distribution Measurement By Sonic Pyrometry", Central Electricity Generating Board, Marchwood Engineering Laboratories, Marchwood, Southampton, England, 1983) discusses the feasibility of acoustic pyrometry using high intensity sparking gaps as acoustic sources and microphones as signal receivers. A standard laboratory oscilloscope was used by Green and Woodham as an instrument to detect the arrival of an acoustic wave and thus determine the flight time of the acoustic wave. The oscilloscope X-axis sweep was triggered the instant an acoustic wave was generated and transmitted. The received signal, which controls the Y-axis motion of the oscilloscope's electron beam, caused the waveform to appear on a display screen. A visual inspection technique was used to determine when a given detection threshold, or received signal magnitude was reached. This technique suffered from the shortcoming that the detection point was not clearly defined. It depended on a visual determination of the point of threshold crossing to know when a signal had arrived. This does not lend itself to continuous temperature measurement. The technique cannot be automated or used in multiple channel applications. Output data was not in a readily usable form. Very high level acoustic signals were required, which in turn required high voltages to generate a high intensity spark as a sound source.
An accurate and practical technique is still thus needed in acoustic pyrometry.
Acoustic pyrometry is particularly useful to measure temperatures across various parts of a steam generator.
Flue gas temperatures are important operating parameters during boiler start-up and after the boiler goes on-line. During start-up, furnace exit gas temperatures (FEGT) must be continuously monitored to avoid overheating superheater tubes until steam flow is established. At higher operating loads, gas temperatures can be monitored at different locations in the furnace, and between tube banks to provide an indication of surface cleanliness.
Poor quality fuels have a tendency toward increased slagging and fouling. This may penalize boiler efficiency through increased flue gas temperature leaving the boiler. It would be advantageous to continuously measure gas temperatures at certain locations in a boiler as a way to monitor heat transfer surface cleanliness.
Water cooled high velocity thermocouple (HVT) probes have been used to measure temperatures above 1000.degree. F. (538.degree. C.). While these have been successful for many years, it becomes impractical to use such probes where boiler width exceeds 40' to 50' (12-27 meters), since the maximum working length of a probe is 24' (7.3 meters). Since HVT probes also determine temperatures only at specific points, point by point temperature measurements must be taken over a very short period of time to measure the average temperature in a certain area of the boiler. These probes are also difficult to maneuver and support.
Optical pyrometers have also been used for several years as a way to take spot readings (primarily in combustion zones). For such a pyrometer to be reliable, however, it must be responsive to a number of optical wavelengths. Some preliminary knowledge of thermal gradients through the gas medium to the point where the temperature measurement is to be made must also be available. Optical pyrometers have not been widely used as continuous measurement devices in boilers. They do not work well below 1,600.degree. F. (871.degree. C.) and would not be useful for monitoring FEGT's during boiler start-up.
Acoustic pyrometry has several advantages over prior systems for measuring high temperatures. One advantage is that the average temperature along a line of sight between the acoustic transmitter and the acoustic receiver can be measured.
For flue gas analysis, using acoustic pyrometers, two primary parameters must be utilized.
The speed of sound through a gas depends on its specific heat ratio, universal gas constant, molecular weight, and absolute temperature as follows: EQU c=(kRT/M).sup.0.5 ( 1)
where:
c--speed of sound, ft/s PA1 k--specific heat ratio, dimensionless PA1 R--universal gas constant, 1545 ft-lb/mol-R PA1 T--absolute temperature, R PA1 M--molecular weight, lb mole PA1 c--speed of sound, ft/s PA1 d--distance over which sound travels, ft PA1 t--flight time of acoustic wave, s PA1 F--gas temperature, deg F PA1 d--distance, ft PA1 B--acoustic constant, (defined in Equation (2) PA1 t--flight time, ms
Equation (1) is typically given to denote the speed of sound. To make it dimensionally consistent, the numerator term within parentheses needs to be multiplied by 32.17 lbm-ft/lbf-s.sup.2.
Equation (1) can be re-written as: EQU c=BT.sup.0.5 ( 2)
where: EQU B=(kR/M).sup.0.5
For air, B typically has a value of 49 ft/s-T.sup.0.5. The only temperature dependent variable is k in Equation (1), but it does not vary significantly over a wide temperature range as shown in Table 1.
TABLE 1 ______________________________________ COMPARISON OF CONSTANTS Temperature Specific Heat Ratio, k Acoustic Constant, B Deg. F. Air Flue Gas Air Flue Gas ______________________________________ 70 1.40 1.37 49.11 48.29 500 1.38 1.35 48.79 47.90 1000 1.34 1.31 48.16 47.25 1500 1.32 1.29 47.78 46.87 2000 1.31 1.28 47.54 46.64 2500 1.30 1.27 47.38 46.48 3000 1.29 1.26 47.26 46.36 ______________________________________
Specific heat values for air are based on a 1.0 percent moisture content by dry weight (lb moisture/lb dry air). Flue gas specific heats are based on 12.0 percent carbon dioxide, 6.0 percent oxygen, and 82.0 percent nitrogen content by dry volume basis with a 5.0 percent moisture content by weight (lb moisture/lb dry gas). All values in Table 1 were calculated in accordance with ASME PTC-11, Fans, 1984.
The speed of sound is determined by measuring the flight time of an acoustic wave then dividing it into the distance travelled. Once the speed of sound is known, the temperature can be computed as shown in the following equations: EQU c=d/t (3)
where:
Combining Equations (2) and (3) gives an expression relating gas temperature to distance and flight time: EQU F=(d/Bt).sup.2 10.sup.6 -460 (4)
where:
U.S. Pat. No. 3,137,169 to Clement et al utilizes a spark gap to produce acoustic waves which are used within a probe to measure the velocity of sound and thus give an indication of temperature. Other patents which teach the use of sound propagated within a probe to measure the velocity of the sound and thus give an indication of temperature are U.S. Pat. Nos. 3,399,570 to Pirlet; 3,580,076 to Mobsby; 3,595,082 to Miller, Jr; 4,005,602 to Wilkie; 3,350,942 to Peltola; 3,534,609 to Grenfell et al; and 3,538,750 to Lynnworth. The last three references utilize ultrasonic sound rather than audible sound.
Phase shift between a transmitted sound, whether it is audible or ultrasonic, and the received sound is also used as a measurement for velocity and thus usable as a measurement for temperature, in U.S. Pat. Nos. 2,934,756 to Kalmus; 4,201,087 and 4,215,575, both to Akita et al; and 4,215,582 to Akita. The use of shifts in frequency of sound to measure temperature is also disclosed in U.S. Pat. Nos. 3,427,881 to Steinberg; 3,451,269 to Johnson; 3,885,436 to Meyer; and 4,020,693 to Ahlgren et al. By sensing the presence of a distinct sound pulse at a receiver, transit times and thus velocity for sound is measured in U.S. Pat. Nos. 4,112,756 to MacLennan et al; and 4,145,922 to Estrada, Jr. et al.
The use of audible and ultrasound in combination with a phase shift meter for measuring temperature fluctuations is disclosed in U.S. Pat. No. 2,834,236 to Pardue et al. A probe using sound waves for measuring temperature is disclosed in U.S. Pat. No. 3,585,858 to Black. The frequency of acoustic waves are also used as a measure of temperature in U.S. Pat. No. 3,769,839 to Innes.
The Doppler shift of sound through the atmosphere is used to measure the temperature of the atmosphere in U.S. Pat. No. 4,222,265 to Ravussin. Ultrasound is used to measure the temperature of living tissue in U.S. Pat. No. 4,452,081 to Seppi. Also, see U.S. Pat. No. 4,469,450 to DiVencenzo which shows the use of sound for measuring temperature.
Several major obstacles exist in using acoustic pyrometry to measure high temperatures in noisy non-homogenous and turbulent environments, such as those existing within a steam generator, boiler or furnace. None of the references cited above suggest any way that an acoustic wave, whether it is ultrasonic or in the audible range, can be sensed across an environment where the noise level may be comparable in amplitude to the audible waves. There is certainly no teaching on how audible waves can be sensed where the noise level may be even higher in amplitude than the audible waves. Some references solve the problem by utilizing a probe containing a homogenous medium through which the sound waves are propagated. Other references utilize acoustic pulses of sufficiently high amplitudes so that they are clearly discernible at a receiver and distinguishable from ambient noise.